Electroconductive powder and process for production thereof

ABSTRACT

By pouring a solution containing hydrolyzable tin and antimony salts into an aqueous dispersion of titanium oxide particles and mixing, the salts are caused to hydrolyze, whereby a uniform and strongly adherent coating layer of antimony-containing tin oxide is formed on the surface of each particle of the titanium oxide. The coated powder thus obtained is non-hygroscopic and non-toxic and exhibits stable electroconductivity. Furthermore, since it can possess excellent whiteness, it is suitable for forming the electroconductive layers of electro-thermosensitive papers and electrostatic recording papers, for use as the antistatic agent of resins, and for other uses.

FIELD OF THE INVENTION

This invention relates to a coated electroconductive powder suitable foruse in applications such as forming electroconductive layers on paperfor reproduction or duplication, such as electro-thermosensitive paperand electrostatic recording paper, and addition to resins to provideantistatic resins.

BACKGROUND OF THE INVENTION

The method of coating an electroconductive powder together with a binderon a substrate such as paper thereby to form thereon anelectroconductive layer for the purpose of forming a paper forduplication such as an electrothermosensitive paper or an electrostaticrecording paper is being widely practiced. The term"electro-thermosensitive paper" is herein used to designate a laminatedcomposite sheet comprising a substrate layer, an electroconductive layersupported thereon, and a thermosensitive color-producing layer disposedon the electroconductive layer. The electroconductive layer is utilizedas one electrode to cooperate with a recording head constituting anotherelectrode in generating Joule heat by which a recording is made on thethermosensitive layer. An electroconductive powder to be used for suchpurposes is required to have, in addition to high and stableelectroconductivity, characteristics such as a white color tone,uniformity of properties such as particle size and composition,excellent dispersibility, nontoxic nature, and low price.

However, copper iodide (CuI) powder, which is being widely used atpresent for the electroconductive layers of electro-thermosensitivepapers, does not afford a perfectly white color tone although it doeshave good electroconductivity. Furthermore, because its production isunstable, it is difficult to produce this copper iodide powder in auniform and stable manner. Still another problem arises from thetoxicity of iodine.

In the case of an electrostatic recording paper, the electroconductivityof its electroconductive layer may be lower than that of anelectro-thermosensitive paper. That is, it may have a high resistance.For this reason, organic electrolytes are being used for theelectroconductive substance. However, because these electrolytes havethe characteristic of adsorbing moisture in the air and in the substratepaper, this adsorbed moisture causes the resistance of theelectroconductive layer to vary, making it difficult to secure stableelectroconductivity. Zinc oxide (ZnO) whose resistance has been loweredby doping with aluminum is also being used, but it has poor resistanceto humidity and lacks stability.

As additives to be added for imparting electroconductivity to resins orfor lowering their tendency to be electrostatically charged, (a) powdersof metals such as Ni, Cu, and Al, (b) Ag powder, and (c) carbon black,carbon fibers, etc., are known. However, oxide films are produced on thesurfaces of powders (a), and their contact resistances increase; Agpowder (b) is expensive; and carbon materials (c) give black resins.Thus, none of these additives can be said to be fully satisfactory as apractical agent for imparting electroconductivity, especially, to awhite resin.

SUMMARY OF THE INVENTION

It is an object of this invention to provide a coated electroconductivepowder which has good and stable electroconductivity and, in accordancewith necessity, is capable of imparting excellent white color tone, andwhich can be produced at a relatively low cost.

I have found from the results of my research that the foregoing objectis achieved through the provision of the electroconductive powder ofthis invention which comprises titanium oxide particles and a coatinglayer of antimony-containing tin oxide applied on the surfaces of thetitanium oxide particles.

It is known that antimony-containing tin oxide possesseselectroconductivity. However, antimony-containing tin oxide powder byitself cannot be produced as a powder having uniform particle size andexcellent dispersibility, and, moreover, it has the disadvantage ofentailing high production cost. Still another difficulty is that, as theantimony content is increased, the color becomes bluish, and the degreeof whiteness decreases.

In contrast, according to this invention, titanium oxide particles areused as nuclei, and their surfaces are coated with antimony-containingtin oxide. As are result, the good dispersibility and high degree ofwhiteness possessed by the titanium oxide particles are utilized, and,by the combination of these features and the surface electroconductivityof the antimony-containing tin oxide, the desired electroconductivecomposite powder is obtained. In the case of this invention, also, somelowering of the degree of whiteness, which occurs with an increase inthe antimony content, cannot be avoided, but the extent of this loweringof the degree of whiteness is much less than that of antimony-containingtin oxide alone because of the excellent whiteness of the titanium oxideparticles. Furthermore, titanium oxide particles have an excellentadhesiveness with respect to antimony-containing tin oxide which cannotbe obtained with other white inorganic particles.

A second object of this invention is to provide an efficient process forproducing the electroconductive powder mentioned above and more fullydescribed hereinafter.

I have found that this object can be achieved by a process wherein anantimony salt and a tin salt are hydrolyzed in the presence of dispersedTiO₂. More specifically, the process of producing electroconductivepowder according to this invention is characterized by the steps ofmixing an aqueous dispersion of titanium oxide particles with a solutioncontaining a hydrolyzable tin salt and a hydrolyzable antimony saltthereby to produce titanium oxide particles coated withantimony-containing tin oxide, and recovering the coated titanium oxideparticles.

The nature, utility, and further features of this invention will be moreclearly apparent from the following detailed description beginning witha consideration of general aspects of the invention and concluding witha number of specific examples of practice illustrating preferredembodiments thereof.

Throughout the following disclosure, quantities, includingconcentrations and proportions, are expressed in percent and parts byweight unless specified otherwise.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings:

FIG. 1 is a graph indicating the relationship between the content ofantimony-containing tin oxide and powder resistivity respectively oftitanium oxide particles coated with antimony-containing tin oxide, andof a mixture of powders of antimony-containing tin oxide and titaniumoxide;

FIG. 2 is a graph indicating the relationship between the antimonycontent in a coating and resistivity of coated powder for coatings ofdifferent thicknesses; and

FIG. 3 is a graph indicating the relationship between powder resistivityof a coated powder and surface resistance of an electroconductive layercontaining that coated powder.

DETAILED DESCRIPTION OF THE INVENTION

As mentioned hereinbefore, the coated electroconductive powder of thisinvention has a structure comprising titanium oxide particles as nucleiand a coating of antimony-containing tin oxide on the surfaces of thetitanium oxide particles.

Titanium oxide (Ti0₂) particles that are generally solid and have aspecific surface area (BET method, N₂ adsorption) in the range of 1 to20 meter² /gram (m² /g) (corresponding to an average particle size of0.07 to 1.4 μm), particularly 1 to 10 m² /g, are preferred. I have foundthat, when the specific surface area is less than 1 m² /g, the TiO₂particles become coarse, and, when they are used to form theelectroconductive layer of a paper for reproduction, the smoothness ofthe paper surface is impaired, and that, when the specific surface areaexceeds 20 m² /g, the aggregation of the particles becomes extensive,and it becomes difficult to obtain a uniform aqueous dispersion in theproduction process described hereinafter.

The function of the TiO₂ particles is not merely to serve as inorganicparticles of white color. The use of the TiO₂ particles is critical inthe sense that they also serve as a substrate of high adhesiveness withrespect to the antimony-containing tin oxide layer. TiO₂ particles ofthe rutile-type crystalline structure are particularly preferable overthose of the anatase-type crystalline structure. At least one reason forthis may be that the antimony-containing tin oxide also becomesstabilized with the rutile-type crystalline structure. Furthermore,titanium oxide particles, as TiO₂, of a purity of 98 percent or higher,particularly 99 percent or higher, are preferred, probably because of arelationship between this purity and retention of the crystallinestructure. Since the coating layer thus obtained has excellentadhesiveness with respect to the substrate TiO₂, it will not peel offwhen the coated particles are subjected to ordinary powder treatments orto a process such as ball-milling.

The antimony (Sb)-containing tin oxide (typically, SnO₂) layer exists asa coating layer adhering intimately or strongly to the surfaces of theTiO₂ particles. This fact has been corroborated by surface analysis byelectron spectroscopy for chemical analysis and also by observation bymeans of an electron microscope. That a uniform coating layer is formedin this manner means that the electroconductivity possessed by theSb-containing tin oxide can be effectively utilized.

In this connection, FIG. 1 is a graph in which powder resistivity valuesare plotted against variation in the quantity of Sb-containing tin oxidewith respect to two kinds of powders. One kind of powder is a coatedpowder comprising particles of TiO₂ of a specific surface area ofapproximately 6 m² /g on which 9.3-percent Sb-containing tin oxide hasbeen formed as coating layers of various thicknesses. The other is apowder mixture of the same TiO₂ particles and particles (specificsurface area of 30 m² /g) of the same Sb-containing tin oxide. As isapparent from this graph, a much lower powder resistivity can beobtained with the coated powder than with the powder mixture for thesame quantity of Sb-containing tin oxide. The powder resistivitiesappearing throughout the disclosure were measured for the powders whilebeing compressed under a pressure of 150 kg/cm² G.

As the quantity of the Sb-containing tin oxide in the coated powderincreases, the powder resistivity, in general, decreases. However, aneven better interrelation with the powder resistivity is obtained by thethickness of the coating layer rather than the quantity of theSb-containing SnO₂ itself. Here, the thickness of the coating layer isdetermined in the following manner.

In the case where the contents of Ti, Sn, and Sb are obtained from anelemental analysis of the coated powder (also, it has been confirmedthat, in the case of the production process described hereinafter, thecorrespondence between the contents of the respective elements and theratio of starting material quantities is also good), calculation iscarried out with the assumption that these metals all exist in the formsof TiO₂, SnO₂, and SbO₂, and the proportion of the coating layerexpressed as x percent is obtained by dividing the sum SnO₂ +SbO₂ by thetotal of SnO₂ +SbO₂ +TiO₂. When the density of the coating layer isassumed to be substantially constant at 6.9 g/cm³, and the specificsurface area of the TiO₂ particles used is denoted by A (m² /g), thethickness t (μm) of the coating layer is given by the followingequation. ##EQU1##

In general, coating layers of thicknesses in the range of 0.001 to 0.07μm as calculated in this manner are used. I have found that with athickness less than 0.001 μm, the intended improvement of theelectroconductivity cannot be obtained. With a thickness exceeding 0.07μm, the rate of improvement in electroconductivity with increase inthickness is not very much; rather, a phenomenon of peeling of thecoating layer tends to occur. Preferred narrower ranges of thisthickness for specific applications vary, of course, since there arenumerous uses for electroconductive powder. For example, in cases whereelectroconductivity is important, as in the use of the powder forforming an electroconductive layer for an electro-thermosensitive paperor in blending with resins, the thickness range of 0.007 to 0.07 μm ismore preferable, whereas in the case where the requirement forelectroconductivity is of relatively less importance, as in the formingof an electrostatic recording layer, the thickness range of 0.001 to0.04 μm is preferable.

The Sb content in the coating layer, in general, is preferably 0.1percent or more in order to obtain the intended improvement inelectroconductivity. Furthermore, in the case where a white coatinglayer is desired, an Sb content of 30 percent or less is preferred.While the Sb can be considered to exist as a mixture of oxides such asSb₂ O₃ and Sb₂ O₅ in the coating layer, it is here assumed that the Sbcontent can be calculated from the formula Sb/(SbO₂ +SnO₂).

The effect of the Sb content in the coating layer on the powderresistivity differs with the thickness of the coating layer. Forexample, FIG. 2 is a graph indicating the interrelation between Sbcontent in the coating layer and powder resistivity when coating layersof different thicknesses are formed on TiO₂ particles having a specificsurface area of 6 m² /g. This graph indicates that, as the coatingthickness is reduced, the Sb content corresponding to minimum powderresistivity increases. In general, as the Sb content increases, thecoating layer tends to turn bluish. For this reason, it can be said thatthe optimum Sb content with respect to each coating layer thickness isthat resulting in the minimum value of the resistivity or a valuesomewhat on the lower side of the minimum value. From the standpoint ofobtaining whiteness and high electroconductivity, the Sb content in thecoating layer is in the range of preferably 0.1 to 20 percent, mostpreferably 1 to 15 percent.

The process according to this invention, which is a preferred process ofproducing the above described electroconductive powder of thisinvention, will now be described with respect to its preferred mode.

According to this invention, first, an aqueous dispersion of the TiO₂ isobtained. While the concentration of the TiO₂ in this dispersion is notespecially limited provided that a uniform dispersion can be obtained,it is preferably in the range of 10 to 500 g/l.

Separately, a solution containing a hydrolyzable Sn salt and ahydrolyzable Sb salt is prepared. For these Sn and Sb salts, chloridesare preferably used for ease in handling, and low price, but, inaddition, their other halides, sulfates, nitrates, oxalates, andacetates are also useable.

For the solvent of the above mentioned solution, any solvent whichdissolves the above enumerated salts and, at the same time, suppresseshydrolysis thereof can be used. For example, solvents such as loweralcohols having 1 to 5 carbon atoms, ketones such as acetone andmethylethyl ketone, and aqueous solutions of acids containing counterions of the above mentioned salts, for example, hydrochloric acid, in aconcentration (for example, 73 g/l or higher in the case of hydrochloricacid) sufficient to suppress hydrolysis of the above mentioned salt canbe used. Of these solvents, ethanol, methanol, and an aqueous solutionof hydrochloric acid are most preferable because of their low prices andfacility in handling.

The concentration of the salt in the solution, in the case of achloride, as a particular example, tin tetrachloride (SnCl₄), ispreferably in the range of 78 to 783 g/l. I have found that, when thisconcentration is less than the lower limit, the SnO₂ content is totallyinsufficient, which gives rise to a lowering of the rate of depositionof SnO₂ on the surfaces of the TiO₂ particles, a lengthening of theprocessing time, and a lowering of the process efficiency. On the otherhand, when the upper limit of this concentration is exceeded, thedeposition of the SnO₂ on the TiO₂ particles becomes nonuniform, and itbecomes difficult to form a coating layer of uniform thickness.

The concentration of the antimony chloride (SbCl₃), as a result of itsrelation to the SnO₂ concentration, is so set that the above mentionedrange of concentration of the Sb in the coating layer is obtained. Fromthis standpoint, a preferable SbCl₃ concentration is 0.085 to 410 g/l,particularly 0.854 to 150 g/l.

It is also possible, depending on the necessity, to use a tin chlorideother than SnCl₄ and an antimony chloride other than SbCl₃ since theyare also hydrolyzable. The quantities in which these other chlorides areused are so determined that their metal concentrations will conform tothe above described standard.

The solution containing the hydrolyzable salts obtained in the abovedescribed manner and the aforedescribed TiO₂ dispersion are then mixed,and the salts are hydrolyzed thereby to form a coating layer ofSb-containing SnO₂ on the TiO₂ particles. During this step, in order toobtain a uniform coating layer, it is preferable to feed the solutioncontaining the hydrolyzable salts into the TiO₂ dispersion as thisdispersion is agitated. It is also preferable during this step to keepthe TiO₂ dispersion heated at a temperature of, for example, 60° to 100°C. in order to promote the hydrolysis and also to result in a coatinglayer with a strong adhesiveness.

According to my experience, since substantially the entire quantity ofthe hydrolyzable Sn salt and Sb salt contained in the added solution ishydrolyzed to produce the Sb-containing SnO₂ layer, the thickness of thecoating layer can be controlled by controlling the addition quantity orconcentration of the solution. Furthermore, in order to promote thehydrolysis, it is also possible to add an alkaline substance such as,for example, caustic soda, caustic potassium, or ammonia, into thehydrolysis bath in a quantity within a range for maintaining the aciditythereof.

In the process of this invention, it is necessary that the Sn salt andthe Sb salt hydrolyze simultaneously in the presence of dispersedparticles of TiO₂. Accordingly, it is preferable to make both the Snsalt and the Sb salt into a common solution and to add this into theTiO₂ dispersion, but it is also possible to prepare an Sn salt solutionand an Sb salt solution separately and to add these solutionssimultaneously into the dispersion. In this case, the solutioncontaining both the Sn salt and the Sb salt to be mixed with thedispersion is formed in situ in the hydrolysis system.

The coated particles obtained in this manner are next separated out by acommonly used solid-liquid separation measure such as precipitation orfiltration and, according to necessity, are subjected to treatment suchas washing with water, drying, and crystal growing by heating, therebyyielding the coated electroconductive particles of the invention. Thepowder resistivity of the electroconductive powder can be adjustedwithin a range of 1 Ωcm to 1 MΩcm by controlling the coating layerthickness or/and the Sb content within the coating layer.

As mentioned hereinbefore, the electroconductive powder of thisinvention can be used for forming electroconductive layers ofreproducing paper such as electro-thermosensitive paper andelectrostatic recording paper by applying this powder together with abinder as a coating on a substrate sheet material, and for impartingantistatic properties by blending the powder with a resin or coating thepowder on a resin film. In addition, this powder is useable for almostall applications wherein electroconductive powders are used, such aspowder for electrodes and electroconductive glaze for ceramics. Thiselectroconductive powder is particularly useful where it can fullyexhibit its advantageous characteristics as an electroconductive powderhaving excellent whiteness, nonhygroscopicity, nontoxicity, andstability.

FIG. 3 is a graph wherein the surface resistances of electroconductivelayers are plotted against powder resistivity. These electroconductivelayers were formed by coating, in each layer, 75 parts of anelectroconductive powder together with 25 parts of a respective binderon plain paper of a thickness of 10μ (dry). The electroconductivepowder, wherein the specific surface area of the TiO₂ was 6 m² /g, had,for respective electroconductive layers, various powder resistivitiesobtained by varying the coating layer thickness or the Sb content in thecoating layer. The binder was a vinyl chloride/vinyl acetate copolymer.The above mentioned surface resistance of each electroconductive layeris the resistance between a pair of electrodes, each of 1-cm length,spaced 1 cm apart and placed in contact with the surface of theelectroconductive layer so as to define a square of 1 cm² areatherebetween, this resistance being expressed in terms of Ω/□. As isapparent from this graph, surface resistances of 10⁴ to 10⁵ Ω/□ requiredfor electro-thermosensitive papers and surface resistances of 10⁷ to10¹⁰ Ω/□ required for electrostatic recording papers are fully covered.

The foregoing disclosure is concerned principally with a whiteelectroconductive powder and the process of producing the same. However,the electroconductive powder of this invention can be rendered into acolored electroconductive powder by coloring it after production thereofwith a dye or a pigment or by adding a dye or a pigment into the abovedescribed hydrolysis bath, as will be apparent to those skilled in theart. Furthermore, it is also possible, of course, to admix theelectroconductive powder together with a coloring agent into acomposition with which the electroconductive powder is to be blended.

In order to indicate more fully the nature and utility of thisinvention, the following examples of practice thereof are set forth, itbeing understood that these examples are presented as illustrative only,and that they are not intended to limit the scope of the invention.

EXAMPLE 1

30 Grams (g) of a TiO₂ powder having a specific surface area of 5 m² /gwas added to 800 cc of water. The resulting mixture was heated to andheld at a temperature of 90° C., at which it was agitated thereby toprepare a heated suspension in which the above described TiO₂ powder wasuniformly dispersed. Into this heated suspension, a solution obtained bydissolving 75 g of SnCl₄ and 2.46 g of SbCl₃ in 200 cc of ethanol wasslowly fed over a period of 2 hours thereby to deposit and form acoating layer comprising Sb-containing SnO₂ on the surfaces of the TiO₂particles. The coated particles thus obtained were separated byfiltration and washed. Then, in order to further improve thecrystallinity of the coating layer, the particles were held in air at atemperature of 500° C. for 2 hours thereby to produce a coated powder ofthis invention.

This coated powder on each particle thereof, had a coating layer ofwhite color comprising 2.9 percent of Sb and a remainder essentially ofSnO₂ and having a uniform layer thickness of 0.042 μm, and was a powderof good electroconductivity having a resistivity of 10 Ωcm.

EXAMPLE 2

100 g of a TiO₂ powder having a specific surface area of 3 m² /g wasadded to 1,000 cc of water. The resulting mixture was heated to and heldat a temperature of 90° C., at which it was agitated thereby to preparea heated suspension in which the above described TiO₂ powder wasuniformly dispersed. Into this heated suspension, a solution obtained bydissolving 25 g of SnCl₄ and 3.28 g of SbCl₃ in 75 cc of ethanol wasslowly dropped over 1 hour. Thereafter, by the same procedure and underthe same conditions as in Example 1, a coated powder of this inventionwas produced.

The coated powder was found to be a powder of good electroconductivityhaving, on each particle thereof, a white coating layer comprising 10.3percent of Sb and a remainder essentially of SnO₂ and having a uniformlayer thickness of 0.007 μm. Moreover, this coated powder had goodelectroconductivity and a resistivity of 380 Ωcm.

EXAMPLE 3

10 g of a TiO₂ powder having a specific surface area of 3 m² /g wasadded to 200 cc of water. The resulting mixture was heated to and heldat a temperature of 90° C., at which it was agitated thereby to preparea heated suspension. Two other lots of the same heated suspension wereprepared in the same manner. Separately, three different solutions wereprepared by dissolving lots of 0.41 g, 0.82 g, and 1.64 g of SbCl₃,respectively, together with 25 g of SnCl₄ in respective 75 cc ofethanol. Each of these solutions was slowly fed over 1 hour into arespectively separate lot of the above described heated suspensionthereby to cause the ethanol to evaporate and cause hydrolysis andthereby to cause a coating layer comprising Sb-containing SnO₂ to bedeposited and formed on the surfaces of the particles of the TiO₂powder. Thereafter, under the conditions and by the procedure of Example1, three kinds of coated powders of this invention were obtained.

These three coated powders were found to have white coating layersrespectively comprising 1.5 percent, 2.9 percent, and 5.7 percent of Sband a remainder essentially of SnO₂ and all having a uniform layerthickness of 0.069 μm. These coated powders were found to have goodelectroconductivity, having resistivities respectively of 120 Ωcm, 6Ωcm, and 2 Ωcm.

EXAMPLE 4

1,000 g of a TiO₂ powder having a specific surface area of 5 m² /g wasadded to 7,000 cc of water, and the resulting mixture was heated to andheld at 90° C. Then, as this heated mixture was agitated, a solutionprepared by dissolving 150 g of SnCl₄ and 20 g of SbCl₃ in 500 cc ofethanol was slowly fed thereinto over 2 hours thereby to cause a coatinglayer comprising Sb-containing SnO₂ to be deposited on the surfaces ofthe particles of the TiO₂ powder. The coated powder thus obtained wasthen separated by filtration, washed, and then subjected to a heattreatment by holding it in air at a temperature of 500° C. for 2 hoursfor the purpose of improving the crystallinity. Thus, a coated powder ofthis invention was produced.

This coated powder was found to have a white coating layer having acomposition comprising 10 percent of Sb and a remainder essentially ofSnO₂ and having a uniform layer thickness of 0.003 μm. Moreover, thispowder had a resistivity of 1.5 KΩcm.

EXAMPLE 5

1,000 g of a TiO₂ powder having a specific surface area of 7 m² /g wasadded to 7,000 cc of water. The resulting mixture was heated to and heldat a temperature of 90° C., at which, as the mixture was agitated, asolution prepared by dissolving 250 g of SnCl₄ and 17 g of SbCl₃ in1,000 cc of methanol was slowly dropped thereinto over 3 hours.Thereafter, by the procedure and under the conditions of Example 4, acoated powder of this invention was produced.

The coated powder was found to have a white coating layer comprising 6percent of Sb and a remainder essentially of SnO₂ and having a uniformlayer thickness of 0.003 μm and, moreover, to have a resistivity of 2KΩcm.

EXAMPLE 6

A coated powder of this invention was produced under the conditionsspecified in Example 4 except for the use of a solution for feeding intothe TiO₂ /water mixture which was prepared by dissolving 100 g of SnCl₄and 17 g of SbCl₃ in 400 cc if an aqueous 6 N hydrochloric acidsolution.

The coated powder of the invention thus produced was found to have aresistivity of 10 KΩcm and, moreover, to have a white coating layerhaving a composition comprising 13 percent of Sb and a remainder of SnO₂and having a uniform layer thickness of 0.002 μm.

EXAMPLE 7

A coated powder of this invention was produced under the conditions setforth in Example 4 except for the use of a solution prepared bydissolving 75 g of SnCl₄ and 12.5 g of SbCl₃ in 400 cc of acetone as thesolution for feeding into the TiO₂ /water mixture.

This coated powder was found to have a resistivity of 100 KΩcm and,moreover, to have a white coating layer comprising 13 percent of Sb anda remainder of SnO₂ and having a layer thickness of 0.0015 μm.

EXAMPLE 8

100 g of a TiO₂ powder having a specific surface area of 5 m² /g wasadded to 1,000 cc of water. The resulting mixture was heated to and heldat a temperature of 90° C. and agitated and was thus held in a state ofa suspension in which the TiO₂ powder was uniformly dispersed. Into thisheated suspension, a solution prepared by dissolving 86.5 g of SnCl₄ and4.93 g of SbCl₃ in 300 cc of methanol was slowly fed over 2 hoursthereby to cause a coating layer of Sb-containing SnO₂ to be depositedand formed on the surfaces of the particles of the TiO₂ powder. The TiO₂powder thus coated was then separated by filtration, washed, and furtherheld in air at a temperature of 500° C. for 2 hours in order to improvethe crystallinity thereof. Thus a coated powder of this invention wasproduced.

This coated powder was found to have a white coating layer comprising 5percent of Sb and a remainder essentially of SnO₂ and constituting aproportion of 33 percent of the entire powder, and to be a goodelectroconductive powder having a resistivity of 30 Ωcm.

EXAMPLE 9

100 g of a TiO₂ powder having a specific surface area of 3 m² /g wasadded to 1,000 cc of water, and the resulting mixture was heated to andheld at a temperature of 90° C., at which it was agitated thereby toprepare a heated suspension in which the TiO₂ powder was uniformlydispersed. As this heated suspension was maintained in this state, asolution prepared by dissolving 34.6 g of SnCl₄ and 4.17 g of SbCl₃ in200 cc of isopropyl alcohol was slowly added thereto over 1.5 hours.Thereafter, by the procedure and under the conditions set forth inExample 8, a composite powder of this invention was produced.

This composite powder was found to have a white coating layer comprising10 percent of Sb and a remainder essentially of SnO₂ and being in aproportion of 17 percent of the entire powder and, moreover, to be apowder of high electroconductivity having a resistivity of 150 Ωcm.

EXAMPLE 10

100 g of a TiO₂ powder having a specific surface area of 3 m² /g wasadded to 1,000 cc of water. The resulting mixture was heated to and heldat 95° C. while it was agitated thereby to prepare a heated suspension.Two other lots of the same heated suspension were prepared in the samemanner. Separately, three solutions were prepared by dissolving 1.19 g,2.41 g, and 4.94 g, respectively, of SbCl₃ together with 86.5 g ofSnCl₄, each, in 300 cc of butyl alcohol, each. Each of these threesolutions was separately fed slowly into a respective lot of the abovedescribed heated suspension over 2 hours thereby to cause evaporation ofthe butyl alcohol and hydrolysis and thereby to cause a coating layercomprising Sb-containing SnO₂ to be deposited and formed on the surfacesof the particles of the above mentioned TiO₂ powder. Thereafter, threecomposite powders of this invention were produced from the resultingpowders under the conditions and by the procedure specified in Example8.

The particles of these composite powders were found to have whitecoating layers respectively comprising 1.25 percent, 2.5 percent, and 5percent of Sb and remainders each essentially of SnO₂ and, moreover,each constituting a proportion of 33 percent of the entire powder. Thesecomposite powders were further found to have good electroconductivity,having resistivities of 360 Ωcm, 20 Ωcm, and 6 Ωcm.

EXAMPLE 11

1,000 g of a TiO₂ powder having a specific surface area of 7 m² /g wasadded to 7,000 cc of water. The resulting mixture was heated to and heldat a temperature of 90° C., and into this mixture, as it was agitated, asolution prepared by dissolving 75 g of SnCl₄ and 30 g of SbCl₃ in 400cc of acetone was slowly poured over 2 hours thereby to cause a coatinglayer comprising Sb-containing SnO₂ to be deposited on the surfaces ofthe particles of the above mentioned TiO₂ powder. The coated powder thusobtained was separated by filtration, washed, and further subjected to aheat treatment comprising holding the powder in air at a temperature of500° C. for 2 hours for the purpose of improving the crystallinity ofthe powder, whereby a coated powder of the invention was produced.

This powder was found to have a coating layer comprising 25 percent ofSb and a remainder essentially of SnO₂ and having a uniform thickness of0.0013 μm and, moreover, to have a resistivity of 1.0 KΩcm.

I claim:
 1. Electroconductive powder consisting essentially of titaniumoxide particles and a coating layer of a mixture of antimony oxide andtin oxide provided on the surfaces of the titanium oxide particles,wherein said titanium oxide particles have a rutile crystallinestructure, a purity of at least 98 percent, and a specific surface areaof 1 to 20 meter² /gram, and said coating layer has a thickness of 0.001to 0.07 μm and contains 0.1 to 30 percent by weight of antimony. 2.Electroconductive powder according to claim 1 in which the titaniumoxide particles have a specific surface area of 1 to 10 meter² /gram. 3.Electroconductive powder according to claim 1 in which the thickness ofthe coating layer is 0.007 to 0.07 μm.
 4. Electroconductive powderaccording to claim 1 in which the thickness of the coating layer is0.001 to 0.04 μm.
 5. Electroconductive powder according to claim 1 inwhich the coating layer contains 0.1 to 20 percent by weight ofantimony.
 6. Electroconductive powder according to claim 1 in which thecoating layer contains 1 to 15 percent by weight of antimony.